I. Field of the Invention
The present invention relates to a glazing unit having improved vibro-acoustic damping properties, which comprises a vibro-acoustic damping device, and to a method of reducing acoustic and vibratory nuisance in a passenger compartment, in particular a moving passenger compartment such as that of a vehicle, especially an automobile.
Apart from automobiles or other types of vehicle, such as trucks, buses and agricultural machines, the invention applies to any type of locomotion means having a closed or substantially closed passenger compartment, such as airplanes, trains, ships, submarines, etc.
II. Description of Related Art
The glazing units associated with vehicles, especially automobiles, are provided with vibro-acoustic damping means that serve to absorb the vibratory waves propagating through the glazing unit for the purpose of improving acoustic comfort inside the vehicles.
The sources of annoyance in an automobile, whether their origin is mechanical, thermal, visibility-related, etc., have little by little been addressed. However, improvement in acoustic comfort still remains a current problem.
Noise of aerodynamic origin, i.e. created by the friction of the air against the moving vehicle, has itself been at least partly treated at its source: to save energy, shapes have been modified, improving penetration through the air and reducing the turbulence which is itself a source of noise. Among the walls of a vehicle that separate the source of exterior aerodynamic noise from the interior space occupied by passengers, the glazing units are of course the most difficult to deal with.
It is presently known to provide laminated glazing units, the thermoplastic interlayer of which is suitably chosen so as to have improved vibro-acoustic damping properties.
European patent EP B1-0 387 148 thus provides laminated glazing units that achieve good insulation against noise, particularly of aerodynamic origin, i.e. having a high frequency, between 800 and 10 000 Hz.
Furthermore, such laminated glazing units serve to prevent a sudden drop at the critical frequency in transmission loss, this being representative of acoustic insulation. The critical frequency is specific to the composition of the glazing unit (density and Young's modulus of the constituent elements, thicknesses) and corresponds to spatial and frequential coincidence of the flexural waves in the glazing unit with the acoustic waves in the fluid surrounding the glazing unit, such as the air. This critical frequency is typically in the region of 4000 Hz for a glazing unit approximately 3 mm in thickness.
It is at this critical frequency, which lies within the range of frequencies that are particularly audible to the human ear (between 1000 and 6000 Hz), that the noise can therefore be increased. It is desirable therefore to have good insulation performance for noise at this frequency.
An alternative solution (since some glazing units are not laminated) or a complementary solution to the use of laminated glazing units having improved vibro-acoustic properties may consist in joining, to the periphery of the glazing unit and between the glazing unit and the body, a strip having vibro-acoustic damping properties which is fastened both to the glazing unit and to the body and which may or may not consist of a juxtaposition of several damping materials.
The term “damping material” is understood in the rest of the description to mean a viscoelastic material having a loss factor of greater than 0.25, which is thus a material capable of dissipating energy. It may for example belong to the family of polymers.
Patent application WO 04/012952 discloses a strip which, in order to provide such a vibro-acoustic damping property, must have an equivalent real linear stiffness K′eq of at least 25 MPa together with an equivalent loss factor tan δeq of at least 0.25. The equivalent linear stiffness is the equivalent stiffness of the strip per linear meter of strip, the stiffness being characterized by the rigidity (mainly the Young's modulus for work in tension-compression) of the materials of which the strip is made and by the geometry of the strip.
In this type of strip, only the stresses and strains undergone by the materials working in tension-compression in the direction normal to the glazing unit are taken into account, those working in shear being negligible. Indeed, compared to the strip, the body is so rigid that it does not deform and cannot absorb vibratory energy. Only the strip can deform significantly and dissipate mechanical energy by working mainly in tension-compression.
This type of damping strip, by also constituting the peripheral joint by which the glazing unit is bonded, is therefore suitable as it provides the connection between the glazing unit and the body. However, in a vehicle not all the glazing units are fixed over their entire periphery to the body—some of them, such as opening side glazing units, are movable. Thus, this peripheral bonding seal configuration acting as damping means cannot be suitable for these side glazing units.